This invention relates to integrated circuit gate drivers and more particularly to such drivers for driving high side power MOSFETs or IGBTs, and to a novel planar MOSFET and integrated series connected Schottky diode.
Integrated circuit MOSFET drivers are well known, for driving the low side and/or high side MOSgated device of a power control circuit. Thus high side drivers are known for controlling the turn on and turn off of a power MOSFET which then permits the connection of electrical power to a load. High side drivers of this kind are known for example, as the IR2015 chip sold by International Rectifier Corporation of El Segundo, Calif.
Such chips will typically consist of a single silicon chip which has a first plurality of control devices integrated in its main body, which is at ground potential, and will also have a second plurality of control devices contained within a high side floating well which is at a high potential relative to ground. The chip will have a number of input pins, including Vcc (control voltage), an input control pin, a comm (or ground) pin, all connected to components in the low voltage portion of the chip and all referenced to ground.
The output to the gate of high side switch (MOSFET or IGBT) can be at a high voltage, so that the input signal to the input pin must be level shifted up. This is commonly done by circuitry in the floating high side well in the integrated circuit chip. The high side circuit xe2x80x9cfloatsxe2x80x9d at the potential of the Vs pin, which is normally connected to the source of the high side switch (MOSFET or IGBT). The output pin HO is connected to the gate of the high side switch to be driven and it provides the drive signal. The voltage difference between the voltages on the Vb and Vs pin provides the supply for the floating high side circuit within the integrated circuit. There are many ways in which the Vbs floating supply can be generated; the bootstrap technique being the simplest and least expensive. In this technique the supply is formed by a high voltage diode and capacitor as shown in FIG. 1 to be later described in detail. This invention is primarily aimed at applications in which the bootstrap technique is used.
When Vs in FIG. 1 is at ground potential the bootstrap capacitor 36 is charged through the bootstrap diode 35 from the 15V Vcc supply. Once this capacitor is fully charged, it retains its charge even when the Vs pin floats to a high voltage, because the bootstrap diode 35 becomes reversed biased. The bootstrap capacitor 36 provides supply current for the high side circuit as well as the gate charge necessary to turn ON the external MOSFET to be driven. However, the bootstrap capacitor 36 must be refreshed by some means before it is discharged significantly.
If the high side switch drives a resistive or inductive load, the bootstrap capacitor 36 is easily refreshed by simply turning the switch off periodically and waiting for the Vs potential to drop to ground (Comm) potential through the load. Once the Vb potential reduces to 0.7V below Vcc the bootstrap diode 35 conducts and re-charges the bootstrap capacitor.
Additionally, in a half bridge circuit the bootstrap capacitor 36 is charged by turning the high side switch (MOSFET or IGBT) off and turning the low side switch (MOSFET or IGBT) on, thus connecting Vs to ground. If the Vb potential is significantly below Vcc the bootstrap diode conducts and refreshes the capacitor.
In absence of resistive (or inductive) loads or a synchronized low side switch, the Vs potential may not automatically drop to ground potential when the high side switch is turned off. In this situation it is desirable to add an internal high voltage MOSFET to the gate driver IC which will connect Vs to ground in order to refresh bootstrap capacitor 36. It was found, however, that such an added transistor could not meet the (xe2x88x92)Vs condition which is often experienced in many applications where Vs goes a few volts below ground potential. During such (xe2x88x92)Vs excursions the inherent drain to body diode of the refresh transistor conducts in its forward conduction direction, generating minority carriers. These minority carriers are injected into the control circuit, and some are collected in the high side floating well and by nearby level shift FET drain regions. This results in small amount of drain current, resulting in malfunction of R-S latch used in level shift circuits [see U.S. Pat. No. 5,545,955 (Wood) for such level shift circuits]. Therefore, the output state of the HO pin can change from low to high (or vice versa) without any input signal.
It would be desirable to provide a means to refresh a bootstrap capacitor in the absence of resistive/inductive loads without danger of producing false control signals. It is also desirable in many application of MOSFETs in general, to prevent conduction of its parasitic diode under forward bias and to prevent injection of minority carriers into nearby control circuits.
In accordance with this invention, a Schottky diode is placed in series with the internal high voltage MOSFET which is used to connect Vs pin to ground in order to refresh the bootstrap capacitor. The refresh transistor and the Schottky can be integrated into the chip and the Schottky device can be formed in series with the drain of refresh transistor.
The novel Schottky operates to add an approximately 0.5 volt drop to the VDS(ON) of the refresh transistor during its on state. However, in the reverse direction, the blocking voltage is increased from (xe2x88x92)0.5 volts to up to about (xe2x88x92)8 volts. Thus, the device body diode does not conduct when Vs goes to (xe2x88x92)ve when the body to drain diode would have otherwise started to conduct and inject minority carriers into the high side well.
A novel high voltage FET and Schottky diode is also formed by a novel process in which the vertical conduction FET is a lateral device, and the drain (or source) is connected to Nxe2x88x92 silicon to define the Schottky.